1. Field of the Invention
The present invention relates to a metal gasket designed to be, and more particularly to a metal gasket disposed between a cylinder head and a cylinder block of an internal combustion engine, and formed of a metallic base plate defining a plurality of openings and having beads formed therearound.
2. Description of the Related Art
It is well known that a gasket maybe disposed between connecting members to seal a clearance therebetween. For instance a cylinder head gasket is disposed, between a cylinder head and a cylinder block of an internal combustion engine. With respect to this gasket, a metal gasket formed of a metallic plate has been widely used. For example, a gasket having a metallic base plate which defines a plurality of openings for passing therethrough pressurized gas such as burnt gas, and which forms beads around the openings to ensure a high sealing pressure against the connecting members is known.
Japanese Utility model Laid-Open Publication No. 59-188955 discloses a sealing pressure adjusting plate whose thickness is smaller than the height of the bead, and which is disposed adjacent to the bead, so as to avoid a reduction in sealing pressure due to undesirable plastic deformation of the bead caused by a clamping force applied thereto. Likewise, it is proposed to form a thick portion on the base plate adjacent to the bead. With respect to this thick portion, there are disclosed a thick portion formed by metal plating in Japanese Utility model Laid-Open Publication NO.61-14748, an auxiliary plate adhered or welded on the base plate in Japanese Utility model Laid-Open Publication No. 2-6855, and a coated stopper (corresponding to the thick portion) with plastics or the like impregnated therein for providing a sealing effect in Japanese Patent Laid-Open Publication No. 64-35057. Also, Japanese Utility model Laid-Open Publication No. 64-8556 discloses a cylinder head gasket provided with a thick portion disposed at the inner side of a bead, and provided with a soft sealing member disposed at a border portion at the outer side of the bead.
As for a spacer such as the above-described sealing pressure adjusting plate, thick portion or the like, which will adjust the bead to be compressed and deformed for functioning as a sealing member, it is preferable to employ a material having a sealing property so as to function as a sealing member itself. In view of various factors such as assembling ability, thickness control, manufacturing cost and the like, it is preferable to secure a metallic ring around the peripheral portion of the opening defined in the base plate.
However, if the metallic ring is secured to the base plate by welding or the like, the base plate will possibly be distorted. The base plate is to be further provided with the sealing member, as described above, so that if the base plate is distorted, it will be difficult to deposit the sealing member at an appropriate position, and the productivity will be decreased.
With respect to the metal gasket as shown in FIG. 6 of the publication No. 64-35057, wherein a couple of base plates of the same configuration forming beads are disposed with the tip ends of the beads facing each other, it is possible to decrease a displacement of the bead of each base plate, so that its anti-fatigue property will be improved. In the publication No. 64-35057, a stopper is formed by spraying and then impregnated with resin, so that it is different from the metallic ring to be secured in terms of property, problems in manufacturing and assembly, or the like. If the metallic ring is substituted for the stopper disclosed in the publication, the sealing member will be formed on the base plate, to which the metallic ring is secured, around the outer periphery of the bead at the protruded side thereof. Therefore, when the sealing member is disposed on the base plate, the same problem as described above will be caused. In addition, since it has been so arranged to dispose an intermediate plate, more material will be necessitated and the thickness as a whole will become greater, in comparison with the art preceding the above publication. As a countermeasure, it is not appropriate to merely remove the intermediate plate, as can be understood in view of the publication in which any structure without the intermediate plate has not been disclosed.
Japanese Utility model Laid-Open Publication No. 61-41960 discloses a metal gasket which deposits on a base plate a vulcanized first coating layer and further deposits a non-vulcanized second coating layer to provide two coating layers. Also, Japanese Utility model Laid-Open Publication No. 1-146064 discloses a metal gasket having a base plate, on which an unvulcanized coating layer including a vulcanizing agent is deposited. In the latter publication, a soft coating layer which contains no vulcanizing agent is defined as a non-vulcanized coating layer, and the one, which contains the vulcanizing agent but which has not been vulcanized, is defined as an unvulcanized coating layer. Also, in the Publication No. 61-41960, the terms "vulcanized" and "non-vulcanized" are defined the same as above.
Japanese Patent Laid-Open Publication No. 64-35057 discloses a metal gasket which overlays two base plates with beads formed thereon and an intermediate plate disposed therebetween, and discloses, as an embodiment, a metal gasket having two base plates disposed with the tip ends of the beads facing each other with the intermediate plate disposed therebetween.
If two base plates are disposed with the tip ends of the beads facing each other and that two coating layers are formed on both sides of each base plate as disclosed in Japanese Utility model Laid-Open Publication No. 61-41960, the tip end of each bead will be covered by two coating layers to make the gasket as a whole thicker by the total thickness of the coating layers. If the coating layer containing the vulcanizing agent is formed on each base plate, as disclosed in Japanese Utility model Laid-Open Publication No. 1-146064, the coating layer will be vulcanized to be hardened depending upon a storage condition, so that a desirable sealing property may not be obtained for the gasket when in use.
When two base plates are arranged such that the tip ends of the beads face each other to provide a metal gasket, the surfaces of the base plates are smooth, different from the surfaces of the cylinder head and cylinder block, so that it may obtain a proper sealing property without the tip end of each bead covered by a coating layer.